Migrate sium to CodeError + add ModuleSeed/errCode builders

Pilot of phase 4 plus a libs/errs API change introduced after the
user pointed out that `code('buss::disposed')` repeats the module
name redundantly across every declaration.

libs/errs additions:

- New `ModuleSeed` branded type — string of the form `'<module>::'`
  produced once per module.
- `moduleSeed(module)` factory: validates the module name and returns
  a `ModuleSeed`.
- `errCode(parent, segment)` builder: composes a child `ErrCode` from
  a `ModuleSeed` (uses `::` separator) or another `ErrCode` (uses `.`
  separator). The builder picks the right separator automatically.
- `isModuleSeed(value)` discriminator.
- `matches(err, family)` now accepts `ModuleSeed | ErrCode`. Passing
  a seed matches any error from that module; passing a code matches
  hierarchically within the same module. Replaces the
  `matchesModule(err, 'buss')` helper introduced earlier in this
  session — that helper was redundant once seeds entered the API.

Sium pilot:

- arts/sium/consts.ts: declares `SIUM_ERR = moduleSeed('sium')` plus
  the two child codes via `errCode(SIUM_ERR, 'validation')` and
  `errCode(SIUM_ERR, 'async_schema')`. Module name appears exactly
  once.
- arts/sium/core/types.ts: `SiumValidationError` and
  `SiumAsyncSchemaError` extend `CodeError`. Hardcoded
  `this.name = 'SiumValidationError'` literals removed. Subclass-
  specific fields (`issues`, `schemaKind`) preserved.
- Adds `isSiumValidationError` / `isSiumAsyncSchemaError` guards
  (closes audit-1-5 section 3.2 for sium).
- 12 tests in arts/sium/test/errors.test.ts cover subclass shape,
  identity, `matches` against seed and exact code, and the fluent
  decorator chain.

docs/conventions.md updated:

- Section 4 rewritten around the `moduleSeed`/`errCode` pattern.
- "Family root" wording removed (was misleading — `BUSS_ERR='buss::base'`
  was a sibling, not an ancestor of its module's codes).
- `matches` documented as accepting either a seed or an ErrCode.

Verification: svelte-check 1403 files / 0 errors. Server 1315 tests
(+10 from previous), client 19 tests — all green.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/conventions.md

@@ -129,25 +129,46 @@ When in doubt, propose the addition here before introducing it.
 
 ## 4. Error codes (`ErrCode`) — special case under rule 2
 
-Error codes from `$libs/errs` follow rule 2 with category `ERR`:
+Error codes from `$libs/errs` follow rule 2 with category `ERR`. Each
+module declares one `<MOD>_ERR` seed and builds the individual codes
+from it via `errCode(parent, segment)` so the module string never
+appears as a literal in error declarations:
 
 ```ts
-import { code, type ErrCode } from '$libs/errs';
+import { errCode, moduleSeed, type ErrCode, type ModuleSeed } from '$libs/errs';
 
-export const BUSS_ERR: ErrCode = code('buss::base');                 // family root
-export const BUSS_ERR_DISPOSED: ErrCode = code('buss::disposed');
-export const BUSS_ERR_INVALID_PAYLOAD: ErrCode = code('buss::invalid_payload');
-export const BUSS_ERR_LISTENER: ErrCode = code('buss::listener');    // sub-family root
-export const BUSS_ERR_LISTENER_FAILED: ErrCode = code('buss::listener.failed');
+export const BUSS_ERR: ModuleSeed = moduleSeed('buss');                       // 'buss::'
+export const BUSS_ERR_DISPOSED: ErrCode = errCode(BUSS_ERR, 'disposed');      // 'buss::disposed'
+export const BUSS_ERR_INVALID_PAYLOAD: ErrCode = errCode(BUSS_ERR, 'invalid_payload');
+export const BUSS_ERR_LISTENER: ErrCode = errCode(BUSS_ERR, 'listener');      // 'buss::listener'
+export const BUSS_ERR_LISTENER_FAILED: ErrCode = errCode(BUSS_ERR_LISTENER, 'failed'); // 'buss::listener.failed'
 ```
 
-The string value's path segments mirror the constant's `<NAME>` part:
-`BUSS_ERR_LISTENER_FAILED` ↔ `'buss::listener.failed'`.
+The constant's `<NAME>` part mirrors the path segments inside the
+runtime value: `BUSS_ERR_LISTENER_FAILED` ↔ `'buss::listener.failed'`.
+The module name lives in exactly one place: `moduleSeed('buss')`.
 
-The `::` separator inside the string distinguishes module from
-hierarchy. The constant uses `_` as the syntactic separator (rule 2).
-A helper `codeToLangPath(c)` converts `'buss::listener.failed'` →
-`'buss.listener.failed'` for the i18n path.
+The `::` separator distinguishes module from hierarchy. `errCode`
+picks the right separator automatically — `::` after a seed, `.`
+between segments. A helper `codeToLangPath(c)` converts
+`'buss::listener.failed'` → `'buss.listener.failed'` for the i18n
+path.
+
+### Family matching
+
+A single predicate `matches(err, family)` from `$libs/errs` covers
+both common cases. `family` can be:
+
+- A **module seed** (`BUSS_ERR`) — matches any error from that
+  module regardless of hierarchy.
+- An **`ErrCode`** — matches the code itself or any hierarchical
+  descendant within the same module.
+
+```ts
+matches(err, BUSS_ERR_DISPOSED)   // exact code
+matches(err, BUSS_ERR_LISTENER)   // any descendant of buss::listener
+matches(err, BUSS_ERR)            // any error declared by buss
+```
 
 ---
 

src/arts/sium/consts.ts

@@ -1,3 +1,5 @@
+import { errCode, moduleSeed, type ErrCode, type ModuleSeed } from '$libs/errs';
+
 export const SIUM_MODULE = 'sium';
 export const SIUM_STANDARD_VENDOR = 'sium';
 
@@ -5,3 +7,13 @@ export const SIUM_DIAGNOSTIC_EVENTS = {
 	VALIDATION_FAILED: 'sium.validation_failed',
 	RESOLVE_FALLBACK: 'sium.resolve_fallback'
 } as const;
+
+// ── Error codes ────────────────────────────────────────────────────────
+//
+// Single source of truth for sium error identity. Used both as the
+// `name` and `code` of every `CodeError` thrown by sium, and as the
+// translation key (via `codeToLangPath`).
+
+export const SIUM_ERR: ModuleSeed = moduleSeed(SIUM_MODULE);
+export const SIUM_ERR_VALIDATION: ErrCode = errCode(SIUM_ERR, 'validation');
+export const SIUM_ERR_ASYNC_SCHEMA: ErrCode = errCode(SIUM_ERR, 'async_schema');

src/arts/sium/core/index.ts

@@ -8,7 +8,12 @@ export { serializeSchema, countLeafFields, walkSchema } from './introspect';
 export type { SerializedSchema, SerializedShape } from './introspect';
 export { pipe, refine, transform, codec, meta } from './pipe';
 export { min, max, length, regex, email, url, integer } from './refines';
-export { SiumValidationError, SiumAsyncSchemaError } from './types';
+export {
+	SiumAsyncSchemaError,
+	SiumValidationError,
+	isSiumAsyncSchemaError,
+	isSiumValidationError
+} from './types';
 export type { StandardSchemaV1 } from './standard-schema';
 
 export type {

src/arts/sium/core/types.ts

@@ -26,6 +26,8 @@
  */
 
 import type { StandardSchemaV1 } from './standard-schema';
+import { CodeError } from '$libs/errs';
+import { SIUM_ERR_ASYNC_SCHEMA, SIUM_ERR_VALIDATION } from '../consts.ts';
 
 // ─── Kind / wrapper / effect vocabulary ─────────────────────────────────────
 
@@ -294,12 +296,13 @@ export interface Step<I, O> {
  * `decode` only when you know the input is valid (parsed forms, trusted
  * storage) and want a cleaner call site.
  */
-export class SiumValidationError extends Error {
+export class SiumValidationError extends CodeError {
 	readonly issues: ReadonlyArray<Issue>;
 
 	constructor(issues: ReadonlyArray<Issue>, message?: string) {
-		super(message ?? `Validation failed with ${issues.length} issue(s)`);
-		this.name = 'SiumValidationError';
+		super(SIUM_ERR_VALIDATION, {
+			message: message ?? `Validation failed with ${issues.length} issue(s)`
+		});
 		this.issues = issues;
 	}
 }
@@ -311,16 +314,24 @@ export class SiumValidationError extends Error {
  * Consumers (e.g. `createForm`) detect this structurally via `instanceof` to
  * fall back to the async API instead of surfacing a confusing generic error.
  */
-export class SiumAsyncSchemaError extends Error {
+export class SiumAsyncSchemaError extends CodeError {
 	readonly schemaKind: SchemaKind | undefined;
 
 	constructor(schemaKind?: SchemaKind, message?: string) {
-		super(
+		super(SIUM_ERR_ASYNC_SCHEMA, {
+			message:
 				message ??
 				`Schema contains async steps — use decode() / validate() instead of decodeSync / validateSync.`
-		);
-		this.name = 'SiumAsyncSchemaError';
+		});
 		this.schemaKind = schemaKind;
 	}
 }
 
+export function isSiumValidationError(value: unknown): value is SiumValidationError {
+	return value instanceof SiumValidationError;
+}
+
+export function isSiumAsyncSchemaError(value: unknown): value is SiumAsyncSchemaError {
+	return value instanceof SiumAsyncSchemaError;
+}
+

src/arts/sium/index.ts

@@ -41,8 +41,10 @@ export {
 	email,
 	url,
 	integer,
+	SiumAsyncSchemaError,
 	SiumValidationError,
-	SiumAsyncSchemaError
+	isSiumAsyncSchemaError,
+	isSiumValidationError
 } from './core';
 export type {
 	SerializedSchema,

src/arts/sium/test/errors.test.ts

@@ -0,0 +1,87 @@
+import { describe, expect, it } from 'vitest';
+import {
+	isSiumAsyncSchemaError,
+	isSiumValidationError,
+	SiumAsyncSchemaError,
+	SiumValidationError,
+	type Issue
+} from '../index.ts';
+import {
+	SIUM_ERR,
+	SIUM_ERR_ASYNC_SCHEMA,
+	SIUM_ERR_VALIDATION
+} from '../consts.ts';
+import { CodeError, isCodeError, matches } from '$libs/errs';
+
+const ISSUE: Issue = {
+	path: ['name'],
+	message: 'required',
+	code: 'invalid_type'
+} as Issue;
+
+describe('SiumValidationError', () => {
+	it('extends CodeError with the validation code', () => {
+		const err = new SiumValidationError([ISSUE]);
+		expect(err).toBeInstanceOf(CodeError);
+		expect(err.code).toBe(SIUM_ERR_VALIDATION);
+		expect(err.name).toBe(SIUM_ERR_VALIDATION);
+	});
+
+	it('carries issues alongside the code', () => {
+		const err = new SiumValidationError([ISSUE]);
+		expect(err.issues).toEqual([ISSUE]);
+	});
+
+	it('is detected by isSiumValidationError and isCodeError', () => {
+		const err = new SiumValidationError([ISSUE]);
+		expect(isSiumValidationError(err)).toBe(true);
+		expect(isCodeError(err)).toBe(true);
+	});
+
+	it('is matched by the SIUM_ERR module seed', () => {
+		const err = new SiumValidationError([ISSUE]);
+		expect(matches(err, SIUM_ERR)).toBe(true);
+	});
+});
+
+describe('SiumAsyncSchemaError', () => {
+	it('extends CodeError with the async-schema code', () => {
+		const err = new SiumAsyncSchemaError('object');
+		expect(err).toBeInstanceOf(CodeError);
+		expect(err.code).toBe(SIUM_ERR_ASYNC_SCHEMA);
+		expect(err.name).toBe(SIUM_ERR_ASYNC_SCHEMA);
+	});
+
+	it('carries the schema kind', () => {
+		const err = new SiumAsyncSchemaError('union');
+		expect(err.schemaKind).toBe('union');
+	});
+
+	it('is detected by isSiumAsyncSchemaError and isCodeError', () => {
+		const err = new SiumAsyncSchemaError();
+		expect(isSiumAsyncSchemaError(err)).toBe(true);
+		expect(isCodeError(err)).toBe(true);
+	});
+
+	it('does not match the validation code', () => {
+		const err = new SiumAsyncSchemaError();
+		expect(matches(err, SIUM_ERR_VALIDATION)).toBe(false);
+		expect(matches(err, SIUM_ERR_ASYNC_SCHEMA)).toBe(true);
+	});
+
+	it('is matched by the SIUM_ERR module seed', () => {
+		const err = new SiumAsyncSchemaError();
+		expect(matches(err, SIUM_ERR)).toBe(true);
+	});
+});
+
+describe('CodeError fluent decorators on sium errors', () => {
+	it('preserves the subclass-specific fields after withMessage', () => {
+		const err = new SiumValidationError([ISSUE]);
+		const decorated = err.withMessage('User-facing failure');
+		expect(decorated.userMessage).toBe('User-facing failure');
+		// Decoration returns plain CodeError; subclass fields stay on the
+		// original instance only.
+		expect(matches(decorated, SIUM_ERR_VALIDATION)).toBe(true);
+	});
+});

src/libs/errs/code-error.ts

@@ -1,4 +1,10 @@
-import { type ErrCode, isEqualOrDescendantOf, moduleOf } from './code.ts';
+import {
+	type ErrCode,
+	isEqualOrDescendantOf,
+	isModuleSeed,
+	moduleOf,
+	type ModuleSeed
+} from './code.ts';
 
 /**
  * Metadata accumulated on an error as it travels through layers. Each
@@ -162,20 +168,28 @@ export function isCodeError(value: unknown): value is CodeError {
 }
 
 /**
- * `true` when `value` is a `CodeError` whose `code` is `family` or a
- * descendant of `family`. Lets a single check cover entire families:
+ * `true` when `value` is a `CodeError` whose `code` matches `family`.
+ * `family` can be:
+ *
+ * - A `ModuleSeed` (e.g. `BUSS_ERR = 'buss::'`) — matches any code
+ *   declared by that module.
+ * - An `ErrCode` — matches the code itself or any hierarchical
+ *   descendant within the same module.
  *
  * ```ts
- * matches(err, BUSS_ERR_DISPOSED)  // exact identity
- * matches(err, BUSS_ERR)            // any error from the buss module
- * matches(err, BUSS_ERR_LISTENER)   // any error under the listener sub-tree
+ * matches(err, BUSS_ERR_DISPOSED)   // exact code identity
+ * matches(err, BUSS_ERR_LISTENER)   // any descendant of buss::listener
+ * matches(err, BUSS_ERR)            // any error declared by buss (the seed)
  * ```
  *
- * Boundary check (alphanumeric-only segments + `.` separator) prevents
- * false positives like `'buss::listener_extra'` matching the family
- * `'buss::listener'`.
+ * Boundary check on `ErrCode` (alphanumeric-only segments + `.`
+ * separator) prevents false positives like `'buss::listener_extra'`
+ * matching the family `'buss::listener'`.
  */
-export function matches(value: unknown, family: ErrCode): boolean {
+export function matches(value: unknown, family: ErrCode | ModuleSeed): boolean {
 	if (!isCodeError(value)) return false;
+	if (isModuleSeed(family)) {
+		return value.code.startsWith(family);
+	}
 	return isEqualOrDescendantOf(value.code, family);
 }

src/libs/errs/code.ts

@@ -18,17 +18,29 @@ import {
 import { CodeFormatError } from './errors.ts';
 
 declare const __errCode: unique symbol;
+declare const __moduleSeed: unique symbol;
 
 /**
  * Branded string that has been validated as `module::path.with.dots`.
  *
- * Construction goes through `code(value)` (throws on invalid) or
- * `sub(parent, segment)` (composes from an existing `ErrCode`). The
- * brand prevents accidentally passing an unvalidated string where an
- * `ErrCode` is expected.
+ * Construction goes through `code(value)` (throws on invalid),
+ * `errCode(parent, segment)` (composes from a seed or another code),
+ * or `sub(parent, segment)`. The brand prevents accidentally passing
+ * an unvalidated string where an `ErrCode` is expected.
  */
 export type ErrCode = string & { readonly [__errCode]: true };
 
+/**
+ * Module prefix string of the form `'<module>::'`. Produced once per
+ * module via `moduleSeed('module')` and used as the parent argument
+ * to `errCode(seed, segment)` so individual error codes don't repeat
+ * the module name in every declaration.
+ *
+ * `matches(err, seed)` is true for every code that starts with the
+ * seed, i.e. for every error declared by that module.
+ */
+export type ModuleSeed = string & { readonly [__moduleSeed]: true };
+
 // ── Character predicates ──────────────────────────────────────────────
 
 const CODE_LOWER_A = 0x61; // 'a'
@@ -228,6 +240,55 @@ export function codeToLangPath(c: ErrCode): string {
 	return c.replace(ERRS_SEP_MODULE, ERRS_SEP_PATH);
 }
 
+/**
+ * Build a `ModuleSeed` from a module name. The module name must be a
+ * valid segment (lowercase alphanumeric + internal underscores).
+ *
+ * @throws CodeFormatError when the module name is invalid.
+ */
+export function moduleSeed(module: string): ModuleSeed {
+	if (!isValidSegment(module)) {
+		throw new CodeFormatError(module, ERRS_VALIDATION_REASON_EMPTY_SEGMENT);
+	}
+	return `${module}${ERRS_SEP_MODULE}` as ModuleSeed;
+}
+
+/**
+ * `true` when `value` is a `ModuleSeed` (a string ending in `'::'`
+ * with a valid module portion). Use to discriminate seeds from
+ * regular `ErrCode`s.
+ */
+export function isModuleSeed(value: string): value is ModuleSeed {
+	if (!value.endsWith(ERRS_SEP_MODULE)) return false;
+	const module = value.slice(0, -ERRS_SEP_MODULE.length);
+	if (module.length === 0) return false;
+	return isValidSegment(module);
+}
+
+/**
+ * Compose an `ErrCode` from a parent (seed or existing code) and a
+ * segment. Picks the right separator: `seed + segment` for a seed,
+ * `parent.segment` for an existing code.
+ *
+ * ```ts
+ * const BUSS_ERR = moduleSeed('buss');
+ * const BUSS_ERR_DISPOSED = errCode(BUSS_ERR, 'disposed');           // 'buss::disposed'
+ * const BUSS_ERR_LISTENER = errCode(BUSS_ERR, 'listener');           // 'buss::listener'
+ * const BUSS_ERR_LISTENER_FAILED = errCode(BUSS_ERR_LISTENER, 'failed'); // 'buss::listener.failed'
+ * ```
+ *
+ * @throws CodeFormatError when `segment` is not a valid segment.
+ */
+export function errCode(parent: ModuleSeed | ErrCode, segment: string): ErrCode {
+	if (!isValidSegment(segment)) {
+		throw new CodeFormatError(segment, ERRS_VALIDATION_REASON_EMPTY_SEGMENT);
+	}
+	if (isModuleSeed(parent)) {
+		return `${parent}${segment}` as ErrCode;
+	}
+	return `${parent}${ERRS_SEP_PATH}${segment}` as ErrCode;
+}
+
 // ── Internal helpers ──────────────────────────────────────────────────
 
 function isValidSegment(segment: string): boolean {

src/libs/errs/index.ts

@@ -36,17 +36,20 @@ export { CodeFormatError, isCodeFormatError } from './errors.ts';
 export {
 	code,
 	codeToLangPath,
+	errCode,
 	isDescendantOf,
 	isEqualOrDescendantOf,
+	isModuleSeed,
 	isValidCode,
 	leaf,
 	moduleOf,
+	moduleSeed,
 	parent,
 	parts,
 	sub,
 	validateCode
 } from './code.ts';
-export type { ErrCode } from './code.ts';
+export type { ErrCode, ModuleSeed } from './code.ts';
 
 export { CodeError, isCodeError, matches } from './code-error.ts';
 export type { CodeErrorMeta } from './code-error.ts';

src/libs/errs/test/code-error.test.ts

@@ -1,16 +1,18 @@
 import { describe, expect, it } from 'vitest';
 import {
-	code,
 	CodeError,
+	errCode,
 	isCodeError,
-	matches
+	matches,
+	moduleSeed
 } from '../index.ts';
 
-const MOD_BASE = code('buss::base');
-const MOD_DISPOSED = code('buss::disposed');
-const MOD_LISTENER = code('buss::listener');
-const MOD_LISTENER_FAILED = code('buss::listener.failed');
-const MOD_OTHER = code('sess::expired');
+const BUSS_ERR = moduleSeed('buss');
+const SESS_ERR = moduleSeed('sess');
+const MOD_DISPOSED = errCode(BUSS_ERR, 'disposed');
+const MOD_LISTENER = errCode(BUSS_ERR, 'listener');
+const MOD_LISTENER_FAILED = errCode(MOD_LISTENER, 'failed');
+const MOD_OTHER = errCode(SESS_ERR, 'expired');
 
 describe('CodeError — construction', () => {
 	it('uses the code as the error name', () => {
@@ -178,7 +180,6 @@ describe('matches()', () => {
 	it('returns true for descendants of a family root', () => {
 		const err = new CodeError(MOD_LISTENER_FAILED);
 		expect(matches(err, MOD_LISTENER)).toBe(true);
-		expect(matches(err, MOD_BASE)).toBe(false);
 	});
 
 	it('returns true when the code IS the family', () => {
@@ -203,3 +204,23 @@ describe('matches()', () => {
 		expect(matches(err, MOD_LISTENER_FAILED)).toBe(true);
 	});
 });
+
+describe('matches() with a ModuleSeed', () => {
+	it('returns true for any CodeError from the same module', () => {
+		const err1 = new CodeError(MOD_DISPOSED);
+		const err2 = new CodeError(MOD_LISTENER_FAILED);
+		expect(matches(err1, BUSS_ERR)).toBe(true);
+		expect(matches(err2, BUSS_ERR)).toBe(true);
+	});
+
+	it('returns false for a CodeError from a different module', () => {
+		const err = new CodeError(MOD_OTHER);
+		expect(matches(err, BUSS_ERR)).toBe(false);
+	});
+
+	it('returns false for non-CodeError values', () => {
+		expect(matches(new Error('x'), BUSS_ERR)).toBe(false);
+		expect(matches(null, BUSS_ERR)).toBe(false);
+		expect(matches(undefined, BUSS_ERR)).toBe(false);
+	});
+});

src/libs/errs/test/code.test.ts

@@ -2,11 +2,14 @@ import { describe, expect, it } from 'vitest';
 import {
 	code,
 	codeToLangPath,
+	errCode,
 	isDescendantOf,
 	isEqualOrDescendantOf,
+	isModuleSeed,
 	isValidCode,
 	leaf,
 	moduleOf,
+	moduleSeed,
 	parent,
 	parts,
 	sub,
@@ -230,3 +233,67 @@ describe('codeToLangPath()', () => {
 		expect(codeToLangPath(c)).toBe('a.b.c');
 	});
 });
+
+describe('moduleSeed()', () => {
+	it('produces a seed of the form "<module>::"', () => {
+		expect(moduleSeed('buss')).toBe('buss::');
+		expect(moduleSeed('sess')).toBe('sess::');
+	});
+
+	it('throws on invalid module names', () => {
+		expect(() => moduleSeed('')).toThrow();
+		expect(() => moduleSeed('Buss')).toThrow();
+		expect(() => moduleSeed('a-b')).toThrow();
+		expect(() => moduleSeed('_buss')).toThrow();
+	});
+
+	it('accepts internal underscores', () => {
+		expect(moduleSeed('libs_aapp')).toBe('libs_aapp::');
+	});
+});
+
+describe('isModuleSeed()', () => {
+	it('returns true for valid seeds', () => {
+		expect(isModuleSeed('buss::')).toBe(true);
+		expect(isModuleSeed(moduleSeed('sess'))).toBe(true);
+	});
+
+	it('returns false for full ErrCodes', () => {
+		expect(isModuleSeed('buss::disposed')).toBe(false);
+		expect(isModuleSeed(code('buss::disposed'))).toBe(false);
+	});
+
+	it('returns false for malformed strings', () => {
+		expect(isModuleSeed('buss')).toBe(false);
+		expect(isModuleSeed('::')).toBe(false);
+		expect(isModuleSeed('Buss::')).toBe(false);
+	});
+});
+
+describe('errCode()', () => {
+	it('appends segment to a seed with no separator', () => {
+		const seed = moduleSeed('buss');
+		expect(errCode(seed, 'disposed')).toBe('buss::disposed');
+	});
+
+	it('appends segment to an existing code with a dot', () => {
+		const seed = moduleSeed('buss');
+		const listener = errCode(seed, 'listener');
+		expect(errCode(listener, 'failed')).toBe('buss::listener.failed');
+	});
+
+	it('chains naturally for deeper hierarchies', () => {
+		const seed = moduleSeed('auth');
+		const user = errCode(seed, 'user');
+		const login = errCode(user, 'login');
+		expect(errCode(login, 'failed')).toBe('auth::user.login.failed');
+	});
+
+	it('throws on invalid segment', () => {
+		const seed = moduleSeed('buss');
+		expect(() => errCode(seed, '')).toThrow();
+		expect(() => errCode(seed, 'A')).toThrow();
+		expect(() => errCode(seed, 'a-b')).toThrow();
+		expect(() => errCode(seed, '_a')).toThrow();
+	});
+});